1. Field of the Invention
The present invention relates to an X-ray computer tomography apparatus (to be referred to as an X-ray CT hereinafter) for reconstructing a tomographic image on the basis of projection data obtained in many directions about an object to be examined.
2. Description of the Related Art
Image noise in an X-ray CT includes circuit noise and photon noise. Circuit noise becomes conspicuous when the dose of X-rays which are transmitted through an object to be examined and incident on an X-ray detector is relatively small, i.e., the data level is relatively low, but becomes latent when the dose of incident X-rays is relatively large. Photon noise is caused by fluctuations in the number of photons in incident X-rays, and is in inverse proportion to the square root of the number of photons. Therefore, the photon noise decreases in proportion to the dose of incident X-rays.
That is, the circuit noise in image noise becomes dominant when the dose of incident X-rays is relatively small, whereas the photon noise becomes dominant when the dose of incident X-rays is relatively large.
An oversampling method has been proposed as a method of reducing the circuit noise. Generally, in a data acquisition system (DAS), an output from an X-ray detector is integrated by an integrating circuit during an interval in which pulse X-rays are irradiated. The integral value at the end of this interval is converted into a digital signal by an analog/digital converter. The digital signal is then output, as projection data, to a computer system. In contrast to this, in an oversampling method, an output from the integrating circuit is sampled M times in the initial period of the above interval, and the M integral values are averaged. Similarly, in the oversampling method, an output from the integrating circuit is sampled M times in the final period of the above interval, and the M integral values are averaged. The average value obtained in the initial period is then subtracted from the average value obtained in the final period to obtain projection data. However, the integration interval in the oversampling method is substantially shortened to the interval between the time the integral output reaches the initial average value and the time the integral output reaches the final average value. As a result, the availability of X-rays substantially deteriorates. That is, there is a trade-off between a reduction in circuit noise and an improvement in the availability of X-rays, and these requirements cannot be satisfied at the same time.
The sensitivity of an X-ray detector, typically a semiconductor detector, has recently improved. In contrast to this, the broadening of the dynamic range of a DAS which is mainly dependent on the performance of an analog/digital converter is limited, and hence an overflow may occur. Therefore, for example, a polygonal line amplifier is connected to the input terminal of the analog/digital converter to decrease the gain from a proper voltage, thereby satisfying the above requirements. This, however, sacrifices the resolution.